If a communications system includes a Core Network, CN, and a Radio Access Network, RAN, and both the CN and the RAN support so called multiple-operators core network, FULL-MOCN, i.e. more than one PLMN in the CN, then, as part of the process of connecting a mobile station, MS, which also supports FULL-MOCN to its preferred Public Land Mobile Network, PLMN, in the core network, the radio access network, RAN, which connects the mobile station to the core network, CN, needs to be aware of the ID of the preferred PLMN, which is at present a 4 octet long field.
At present, there is no mechanism for an MS to indicate its preferred PLMN ID to the RAN when accessing the PS domain, i.e. when connecting the MS to the CN, although this is needed in order to enable FULL-MOCN in a communications system.
One method for solving this which has been suggested in 3GPP is to introduce a new Radio Link Control/Medium Access Control, RLC/MAC, message for sending a PLMN ID from the MS to the RAN.
This method might work, but would necessitate the introduction of a new message, and would also increase the load on the “air interface”, i.e. the interface between the MS and the RAN.
Another proposed method is to include the PLMN ID in Non-Access Stratum, NAS, messages sent to the CN. Since NAS messages are tunneled via the RAN on their way to the CN and are thus transparent to the RAN, this implies two significant drawbacks with this approach:    1. Extra signaling procedures, new and/or a re-definition of existing messages between the CN and the RAN are needed.    2. The RAN needs to more or less randomly select one of the PLMNs to decode this NAS message. This PLMN could then very well “hijack” the mobile station by ignoring the PLMN ID that was included from the beginning, i.e. by the MS.